As known widely to those skilled in the art, a semiconductor memory device, such as a DRAM, has a temperature sensor for controlling an operation of internal circuits based on an internal temperature. The temperature information produced by the temperature sensor is used to control the self-refresh period through a detection of the surrounding temperature in order that current consumption is reduced in a self-refresh operation or is used for deciding whether a detected temperature is higher than a predetermined temperature.
FIG. 1 is a block diagram illustrating a conventional temperature sensor.
Referring to FIG. 1, the conventional temperature sensor includes a temperature sensing unit 10 to produce a sensing level Vsen by sensing an internal temperature in a semiconductor memory device, a reference level generating unit 12 to set up a reference level Vset, and a comparison unit 14 to produce a temperature detecting signal HOT_FLAG by comparing the sensing level Vsen to the reference level Vset.
The operation of the temperature sensor will be described below.
First, the temperature sensing unit 10 senses an internal temperature of the semiconductor memory device and then produces the sensing level Vsen. As the internal temperature becomes higher, the sensing level Vsen is lowered.
Next, the reference level generating unit 12 sets up the reference level Vset which has a predetermined level. At this time, the voltage level of the reference level Vset is determined by fuse cuttings which are made by a plurality of metal fuses (not shown) included in a control signal generating unit 122. The voltage level setup of the reference level Vset will be described in detail below.
A reference voltage Vref generated by a reference voltage generating unit 120 is divided by a resistance dividing unit 121 into a plurality of divided reference voltages Vdiv<1:2N> which have voltage levels corresponding to the internal temperatures of the semiconductor memory device.
Meanwhile, the control signal generating unit 122 includes a plurality of the metal fuses (not shown) and produces a plurality of control signals CON<1:N> which are selectively enabled according to the combination of the fuse cuttings of the metal fuses. A decoder 123 produces a plurality of select signals SEL<1:2N> by decoding the plurality of control signals CON<1:N>.
A reference voltage select unit 124, which receives the divided reference voltages Vdiv<1:2N> and the select signals SEL<1:2N>, selects one out of the divided reference voltages Vdiv<1:2N> in response to the select signals SEL<1:2N>.
As mentioned above, the reference level Vset produced by the reference level generating unit 12 is determined by the combination of the fuse cuttings of the plurality of metal fuses (not shown) in the control signal generating unit 122. Accordingly, the reference level Vset can be set up to a voltage level, which corresponds to a predetermined temperature, for example, 90° C., in the semiconductor memory device, by adjusting the combination of the fuse cuttings of the plurality of metal fuses (not show) included in the control signal generating unit 122.
When the reference level Vset is set up, the temperature sensor compares the sensing level Vsen to the reference level Vset through the comparison unit 14 and then verifies whether the internal temperature of the semiconductor memory device is higher than the predetermined temperature or not. That is, in the case that the sensing level Vsen is lower than the reference level Vset, the temperature sensor outputs a temperature detection signal HOT_FLAG of a low level through the comparison unit 14 because the internal temperature of the semiconductor memory device is higher than the predetermined temperature. Also, in the case that the sensing level Vsen is higher than the reference level Vset, the temperature sensor outputs a temperature detection signal HOT_FLAG of a high level through the comparison unit 14 because the internal temperature of the semiconductor memory device is lower than the predetermined temperature.
On the other hand, the conventional temperature sensor has a test mode to set up the reference level Vset and the test mode is executed by a test pulse Test_Pulse which is inputted into the control signal generating unit 122. This will be illustrated in more detail below.
First, the control signal generating unit 122, which is configured to receive the test pulse signal Test_Pulse, produces the control signals CON<1:N> which are selectively enabled regardless of the combination of the fuse cuttings of the metal fuses (not shown).
Next, the decoder 123 produces the select signals SEL<1:2N> by decoding the control signals CON<1:N> and the reference voltage select unit 124 outputs the reference level Vset by selecting one of the divided reference voltages Vdiv<1:2N> according to the select signals SEL<1:2N>.
The above-mentioned test mode is continuously carried out until the temperature detection signal HOT_FLAG is at a high level, that is, until the sensing level Vsen is higher than the reference level Vset. Namely, if the reference level Vset, which is selected whenever the test pulse Test_Pulse is applied to the control signal generating unit 122, is lower than the sensing level Vsen, the test pulse Test_Pulse is continuously applied to the control signal generating unit 122 until the temperature detection signal HOT_FLAG is at a high level (that is, until the sensing level Vsen is higher than the reference level Vset).
When the temperature detection signal HOT_FLAG is at a high level, the combination of the fuse cuttings of the metal fuses included in the control signal generating unit 122 is determined by the number of the test pulses Test_Pulse which are applied to the control signal generating unit 122 in order to set up the reference level Vset. That is, the combination of the fuse cuttings of the metal fuses, which is required to set up the reference level Vset, is found out by the number of the test pulses Test_Pulse.
However, since the above-mentioned test mode continuously applies the test pulse Test_Pulse to the control signal generating unit until the reference level Vest corresponding to a predetermined internal temperature is set up, it takes a lot of time to set up the reference level Vest. Further, an additional laser cutting process is required to apply the combination of the fuse cuttings, which is found out through the test mode, to the temperature sensor.